This invention relates generally to air-conditioning systems for use with a motor vehicle, such as an automobile, and more particularly, the present invention relates to an air-conditioning system arranged to selectively recirculate the air in the vehicle compartment or introduce external or fresh air into the compartment.
Most of conventional air-conditioning systems used in motor vehicles are arranged so that the air in the vehicle compartment circulates or external air is taken into the compartment where recirculation mode and external air introduction mode may be manually switched from one to the other. According to an improved air-conditioning system disclosed in Japanese patent pulication No. 47-39674, the above-mentioned switching between two modes is automatically effected in accordance with the degree of pollution of the air to be taken into the compartment. Namely, a gas sensor is provided at an external air inlet to produce an electrical signal indicative of the degree of pollution of the external air, and an actuator is driven in accordance with the signal so that the external air is led into the vehicle compartment only when it is clean. The above-mentioned publication only discloses that mode changing is effected from external air introduction mode to internal air recirculation mode when the amplitude of the electrical signal from the gas sensor exceeds a predetermined value.
Inventors of the present invention made experiments so as to reduce the concept disclosed in the above-mentioned publication into practice. First of all, various gas sensors including semiconductor type sensors were investigated, and it has been concluded that a gas sensor of solid thermal conduction type is suitable for an air-conditioning system of a motor vehicle, considering structure and cost of gas sensors. The gas sensor of this type operates as follows. A metal oxide, such as SnO.sub.2 or the like, is heated up to 300-500 degrees centigrade, and its thermal conductivity increases when adsorbing an inflammable gas. As a result, heat dissipation is facilitated to cause the temperature of the semiconductor to lower so that the resistance of a platinum filament used for heating the semiconductor decreases.
Using such a gas sensor, the degree of pollution of the air exterior to the vehicle compartment is measured with a motor vehicle travelling in city areas and suburbs. The gas sensor is used as an element constituting a bridge circuit, and the difference in voltages is detected to see the difference in pollution degree in city areas and suburbs. According to the experiments, the maximum differnece voltage is usually 50 mV or so. Therefore, the above-mentioned automatic mode changing control is tried in the experiments with a reference voltage set to 10 mV.
When travelling in city areas, exhaust gases from a motor vehicle which goes ahead are detected each time the motor vehicle equipped with the air-condioning system stops at every intersections, and thus the external air introduction mode is automatically changed to the internal air recirculation mode. As soon as the motor vehicle starts moving again, the internal air recirculation mode is changed to the extenal air introduction mode because the level of the output signal from the gas sensor immediately decreases. Such mode changing is repeatedly occurred frequently, and such frequent mode changining occurred even in a traffic jam. This means that external air, which is not clean enough, is apt to be introduced into the vehicle compartment. Especially, when the external air latently polluted due to traffic jams, such unclean air cannot be prevented from entering into the vehicle compartment.
Furtheremore, if the signal level of the output from the gas sensor drastically varies, mode changings are continuously occurred from one to the other. Such repetitive and frequent mode changing is uncomfortable to the vehicle occupants, and sometimes, the vehicle occupants may misunderstand that the air-conditioning system malfunctions. Namely, the inventors of the present invention have realized that the technique of mode switching disclosed in the above-mentioned publication cannot be simply applied to an air-conditioning system of a motor vehicle because desirable operation cannot be expected if the mode changing technique is used as it stands